1. Field of the Invention
The invention relates in general to a new semiconductor material for radiation absorption and detection, and in particular, to a semiconductor material based on compositions of LiM2+GV that exhibit an antifluorite-type ordering, wherein M2+ is a divalent metal (or metals) and GV represents a member (or members) from the Group V elements.
2. Description of the Related Art
Gamma Photon Detection Methods
Traditional approaches to gamma detection involve high-density materials containing elements with large atomic numbers. These materials can be broken into two general classes: [1] scintillators and [2] direct-conversion semiconductors. For both classes, incoming gamma photons interact with the material, depositing energy in the form of energetic primary electrons, which in turn, ionize electron-hole pairs.
In scintillators, a large fraction of these pairs will recombine either directly or at a luminescent site (i.e., an activator dopant or intrinsic defect) to produce photons. Such photon emissions are typically in the visible spectrum. The photons are collected by a photosensor (e.g., photomultiplier tube, photodiode, etc.) and processed by suitable electronics to re-construct the gamma energy spectrum.
In direct-conversion semiconductors, a large fraction of the electron-hole pairs are collected at electrodes via an applied electric field (i.e., electrons migrate to the anode and holes to the cathode). The resulting current pulses are processed by suitable electronics to re-construct the gamma energy spectrum.
Current commercial scintillators include NaI:Tl, CsI:Tl, CsI:Na, Bi4Ge3O12, (Lu,Y)SiO5:Ce, and LaBr3:Ce. Direct-conversion semiconductors include Si, high-purity Ge, HgI2, PbI2, and members from the Cd1-xZnxTe series.
Neutron Particle Detection Methods
Traditional approaches to neutron detection commonly follow one of four available paths: [1] the gaseous containment of 3He or 10BF3 (e.g., proportional counters, ionization/scintillation chambers), [2] thin layers or doping regions containing either 10B or 6Li atoms on or inside silicon diodes, [3] the solid-state incorporation of 6Li in scintillators (e.g., 6LiI:Eu or 6Li-based, Ce-doped silicate glasses), and [4] hydrogen recoil in organic matter (e.g., anthracene, stilbene, liquid/plastic scintillators).
With the exception of [4], all methods rely on a stable isotope of high neutron cross-section (e.g., 3He, 6Li, 10B) to absorb incident neutron radiation. This absorption process induces a nuclear reaction which produces charged heavy particles as by-products:
3He(n,p) reaction:3He + n → 3T + 1pQ = 0.764 MeV6Li(n,α) reaction:6Li + n → 3T + 4αQ = 4.78 MeV10B(n,α) reaction:10B + n → 7Li + 4αQ = 2.792 MeV(Ground State, 6%)10B + n → 7Li* + 4αQ = 2.310 MeV(Excited State, 94%)The 10B(n,α) reaction, however, also produces a 0.478 MeV gamma photon when 7Li* decays to 7Li.
Heavy particle by-products from these neutron capture reactions can be exploited to generate electron-ion pairs in a gas (i.e., [1]) or electron-hole pairs in a solid-state material (i.e., [2] or [3]). Methods [1] and [2] convert such pairs directly into measurable electrical currents, using suitable processing electronics to register the neutron event(s). Method [3] requires, in addition, a photosensor to first convert the photon emissions from the scintillator into electronic pulses.
Method [4] is distinct from [1], [2], and [3] in that it relies on the kinematics of neutron elastic scattering. In this process, incident neutron particles collide with molecularly-bound hydrogen to liberate recoil protons. These recoil protons function in a manner similar to the heavy, charged by-products of the neutron capture reactions above: their energy is transferred to electrons of their host, typically a gas/liquid or polymeric solid. In organic scintillators—the most common embodiment—electron-hole pairs are created which subsequently recombine to generate photons. The photons are collected by a photosensor (e.g., photomultiplier tube, photodiode, etc.) and processed by suitable electronics to register the neutron event.
The first class of neutron detectors (i.e., [1]) represents the dominant and most mature technology sold commercially. The second class of neutron detectors (i.e., [2]) is based on a converter layer of 6Li-containing or 10B-containing material coupled to a silicon diode. This design has evolved from simple planar layers into different 2-D or 3-D variants (e.g., “perforated” silicon, PIN diode pillar elements, etc.) in order to improve neutron sensitivity. Such a trend, however, has come at the expense of increased manufacturing complexity, which in turn, has raised fabrication costs; good production yields have yet to be demonstrated.
The third class of neutron detectors (i.e., [3]) is commercially available as small single crystals (i.e., 6LiI:Eu sizes up to 1 inch), and in larger/custom volumes, as amorphous monoliths (i.e., 6Li-based, Ce-doped silicate glasses). Unfortunately, the crystals have a non-linear energy response, while the glasses exhibit (in addition) poor light output. As a result, both materials are used only for imaging or counting and not spectroscopy.
The last class of neutron detectors (i.e., [4]) is commercially available in bulk/custom sizes (i.e., solid plastics) or in sealed containers (i.e., liquid organics). These detectors require large volumes of hydrogenous liquid or solid plastic, and as such, their embodiments become sensitized to gamma rays. This latter characteristic necessitates additional, costly pulse-shape electronics to discriminate neutron from gamma events. Another disadvantage is that only the first neutron interaction in the material can be measured. Liquid scintillators are also toxic.
The need to counter loose nuclear weapons threats requires detector systems that can sense concealed and/or shielded radiological materials located on individuals, transported by vehicles, housed within cargo containers, and the like. Furthermore, this detection process must be capable of distinguishing special nuclear materials (e.g., weapons-grade U and Pu, gaseous UF6 for enrichment, etc.) from the presence of medical and industrial radioisotopes, as well as from normally-occurring radioactive material (NORM). Thus, there is a need to provide a material that absorbs gamma and/or neutron radiation, and then converts the energy deposited by this radiation into electrical pulses. These signals can then be processed to create an energy spectrum for each radiation type, thereby enabling radioisotope detection and identification.